The present invention relates to a phase shifter used in meshed power supply systems for improving the transmission of energy and for preventing detrimental reactive effects on the power supply system and the generators when high-voltage lines are switched together.
The flow of energy between two system points connected by lines is determined by the phase angle between the voltages at these points and by the impedance between the two points. These relationships are illustrated in FIGS. 1 and 2. In FIG. 1, N1 and N2 designate the two part systems for three-phase current for the electric transfer and distribution of energy which are connected with each other by a line. E1 and E2 designates the voltages on the bus bars of N1 and N2, X the impedance between E1 and E2, or the series impedance of the line if only only line exists, and P the electric power transferred, for which the following applies: EQU P.apprxeq.E1.multidot.E2.multidot.sin .delta./X,
where .delta. is the phase angle between the voltages E1 and E2, see FIG. 2.
In a three-phase transmission arrangement, two of the main problems are the stability of transmission and the regulation of load flow in the system.
Regarding transmission stability, as the phase angle between two part systems increases, the synchronizing torque keeping the generators of the two part systems in step with each other is reduced. This means that even small short-term system disturbances can lead to loss of synchronization between the power stations linked by the transmission lines. As a rule, with the generator control systems used today and the short fault-elimination times which can be achieved with modern circuit breakers, the limit of dynamic stability is at a phase angle of approximately 30.degree. and that of the transient stability even below this value. By phase angle in this case is meant the angle between the voltage vectors at the beginning and at the end of the line. The angle between the voltage vectors of the internal generator voltages is correspondingly greater. Frequently, with long transmission distances not even the natural line performance can be utilized without exceeding the stability limit. For improving the transmission characteristic of lines and simultaneously supporting stability, series capacitors and controllable shunt compensation are used as conventional measures.
Wtih respect to load-flow regulation, the load flow within an electric system is largely self-regulating. If a deficiency of effective power exists in a part system, this immediately results in a trailing phase angle with respect to the other part systems. This, in turn, causes an immediate increase of the flow of effective power in the direction of the part system having the deficiency of effective power.
Difficulties occur then in ring systems, in highly meshed systems and if part systems are connected to each other via several lines of different voltage levels. Since the load flow through a line is self-regulating as a function of phase angle and series impedance of the lines, it can happen that the individual lines are subjected to greatly differing loads. In most cases, this creates unnecessarily high transmission losses and in extreme cases lines can even be overloaded. This is remedied by devices such as, for example, quadrature regulators which introduce an auxiliary voltage having a phase displacement of 90.degree. or also by phase-angle regulators introducing an auxiliary voltage having a phase displacement of 30.degree. or 60.degree..
Prior art phase shifters are described, for example, in IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 5, May 1981, pages 2650-2655. In this description, in the intermediate circuit between an exciter transformer which is fed by the system voltage and which generates an intermediate-circuit voltage, and an auxiliary transformer which generates an auxiliary voltage and the secondary winding of which is connected in series with the power line to be influenced by the power system, a choke coil is provided which is connected either in parallel or in series with the primary winding of the auxiliary transformer. In the former case, an alternating-current switch containing antiparallel-connected thyristors is connected in series and in the second case in parallel with the primary winding of the auxiliary transformer. This makes it possible to add voltage components which are phase shifted by 90.degree. and have a predetermined amplitude according to the inductance of the choke coil to the system phase voltages during predeterminable time intervals. This publication also discloses the generation of an auxiliary voltage from the voltage drop of a thyristor-controlled choke coil.
At a maximum, the amplitude of the alternating voltage supplied to the primary winding of the auxiliary transformer is in this case equal to the amplitude of the output voltage occurring at the secondary winding of the exciter transformer. In order to generate an auxiliary voltage of a certain amplitude, the exciter transformer must be designed to have a relatively high rating.